Commit message: Fix a json_decode crash when depth==0

Summary:
Setting depth=0 is an error, and should result in NULL, but we weren't
checking for it, so in the case of a single, top-level string, we
would reading the -1th element of the stack.

Differential Revision: D19609959

fbshipit-source-id: 04ca1e0965e04b44df2d5c806a73c3da99ff66fb
CVE: CVE-2020-1892
CWE: CWE-125
Vulnerability description: Insufficient boundary checks when decoding JSON in JSON_parser allows read access to out of bounds memory, potentially leading to information leak and DOS. This issue affects HHVM 4.45.0, 4.44.0, 4.43.0, 4.42.0, 4.41.0, 4.40.0, 4.39.0, versions between 4.33.0 and 4.38.0 (inclusive), versions between 4.9.0 and 4.32.0 (inclusive), and versions prior to 4.8.7.

Project: hhvm
File: JSON_parser.cpp

bool JSON_parser(Variant &z, const char *p, int length, bool const assoc,
                 int depth, int64_t options) {
  // No GC safepoints during JSON parsing, please. Code is not re-entrant.
  NoHandleSurpriseScope no_surprise(SafepointFlags);

  json_parser *json = s_json_parser.get(); /* the parser state */
  // Clear and reuse the thread-local string buffers. They are only freed if
  // they exceed kMaxPersistentStringBufferCapacity at exit or if the thread
  // is explicitly flushed (e.g., due to being idle).
  json->initSb(length);
  SCOPE_EXIT {
    constexpr int kMaxPersistentStringBufferCapacity = 256 * 1024;
    if (json->sb_cap > kMaxPersistentStringBufferCapacity) json->flushSb();
  };
  // SimpleParser only handles the most common set of options. Also, only use it
  // if its array nesting depth check is *more* restrictive than what the user
  // asks for, to ensure that the precise semantics of the general case is
  // applied for all nesting overflows.
  if (assoc &&
      options == (options & (k_JSON_FB_LOOSE |
                             k_JSON_FB_DARRAYS |
                             k_JSON_FB_DARRAYS_AND_VARRAYS |
                             k_JSON_FB_HACK_ARRAYS |
                             k_JSON_FB_THRIFT_SIMPLE_JSON |
                             k_JSON_FB_LEGACY_HACK_ARRAYS)) &&
      depth >= SimpleParser::kMaxArrayDepth &&
      length <= RuntimeOption::EvalSimpleJsonMaxLength &&
      SimpleParser::TryParse(p, length, json->tl_buffer.tv, z,
                             get_container_type_from_options(options),
                             options & k_JSON_FB_THRIFT_SIMPLE_JSON)) {
    return true;
  }

  int b;  /* the next character */
  int c;  /* the next character class */
  int s;  /* the next state */
  int state = 0;

  /*<fb>*/
  bool const loose = options & k_JSON_FB_LOOSE;
  JSONContainerType const container_type =
    get_container_type_from_options(options);
  int qchr = 0;
  int8_t const *byte_class;
  int8_t const (*next_state_table)[32];
  if (loose) {
    byte_class = loose_ascii_class;
    next_state_table = loose_state_transition_table;
  } else {
    byte_class = ascii_class;
    next_state_table = state_transition_table;
  }
  /*</fb>*/

  UncheckedBuffer *buf = &json->sb_buf;
  UncheckedBuffer *key = &json->sb_key;

  DataType type = kInvalidDataType;
  unsigned short escaped_bytes = 0;

  auto reset_type = [&] { type = kInvalidDataType; };

  json->depth = depth;
  // Since the stack is maintainined on a per request basis, for performance
  // reasons, it only makes sense to expand if necessary and cycles are wasted
  // contracting. Calls with a depth other than default should be rare.
  if (depth > json->stack.size()) {
    json->stack.resize(depth);
  }
  SCOPE_EXIT {
    if (json->stack.empty()) return;
    for (int i = 0; i <= json->mark; i++) {
      json->stack[i].key.reset();
      json->stack[i].val.unset();
    }
    json->mark = -1;
  };

  json->mark = json->top = -1;
  push(json, Mode::DONE);

  UTF8To16Decoder decoder(p, length, loose);
  for (;;) {
    b = decoder.decode();
    // Fast-case most common transition: append a simple string character.
    if (state == 3 && type == KindOfString) {
      while (b != '\"' &&  b != '\\' && b != '\'' && b <= 127 && b >= ' ') {
        buf->append((char)b);
        b = decoder.decode();
      }
    }
    if (b == UTF8_END) break; // UTF-8 decoding finishes successfully.
    if (b == UTF8_ERROR) {
      s_json_parser->error_code = JSON_ERROR_UTF8;
      return false;
    }
    assertx(b >= 0);

    if ((b & 127) == b) {
      /*<fb>*/
      c = byte_class[b];
      /*</fb>*/
      if (c <= S_ERR) {
        s_json_parser->error_code = JSON_ERROR_CTRL_CHAR;
        return false;
      }
    } else {
      c = S_ETC;
    }
    /*
      Get the next state from the transition table.
    */

    /*<fb>*/
    s = next_state_table[state][c];

    if (s == -4) {
      if (b != qchr) {
        s = 3;
      } else {
        qchr = 0;
      }
    }
    /*</fb>*/

    if (s < 0) {
      /*
        Perform one of the predefined actions.
      */
      switch (s) {
        /*
          empty }
        */
      case -9:
        /*<fb>*/
        if (json->top == 1) z = json->stack[json->top].val;
        else {
        /*</fb>*/
          attach_zval(json, json->stack[json->top].key, assoc, container_type);
        /*<fb>*/
        }
        /*</fb>*/
        if (!pop(json, Mode::KEY)) {
          return false;
        }
        state = 9;
        break;
        /*
          {
        */
      case -8:
        if (!push(json, Mode::KEY)) {
          s_json_parser->error_code = JSON_ERROR_DEPTH;
          return false;
        }

        state = 1;
        if (json->top > 0) {
          Variant &top = json->stack[json->top].val;
          /*<fb>*/
          if (container_type == JSONContainerType::COLLECTIONS) {
            // stable_maps is meaningless
            top = req::make<c_Map>();
          } else {
          /*</fb>*/
            if (!assoc) {
              top = SystemLib::AllocStdClassObject();
            /* <fb> */
            } else if (container_type == JSONContainerType::HACK_ARRAYS) {
              top = Array::CreateDict();
            } else if (container_type == JSONContainerType::DARRAYS ||
                       container_type == JSONContainerType::DARRAYS_AND_VARRAYS)
            {
              top = Array::CreateDArray();
            /* </fb> */
            } else if (
              container_type == JSONContainerType::LEGACY_HACK_ARRAYS) {
              auto arr = staticEmptyDictArray()->copy();
              arr->setLegacyArray(true);
              top = arr;
            } else {
              top = Array::CreateDArray();
            }
          /*<fb>*/
          }
          /*</fb>*/
          json->stack[json->top].key = copy_and_clear(*key);
          reset_type();
        }
        break;
        /*
          }
        */
      case -7:
        /*** BEGIN Facebook: json_utf8_loose ***/
        /*
          If this is a trailing comma in an object definition,
          we're in Mode::KEY. In that case, throw that off the
          stack and restore Mode::OBJECT so that we pretend the
          trailing comma just didn't happen.
        */
        if (loose) {
          if (pop(json, Mode::KEY)) {
            push(json, Mode::OBJECT);
          }
        }
        /*** END Facebook: json_utf8_loose ***/

        if (type != kInvalidDataType &&
            json->stack[json->top].mode == Mode::OBJECT) {
          Variant mval;
          json_create_zval(mval, *buf, type, options);
          Variant &top = json->stack[json->top].val;
          object_set(json, top, copy_and_clear(*key),
                     mval, assoc, container_type);
          buf->clear();
          reset_type();
        }

        /*<fb>*/
        if (json->top == 1) z = json->stack[json->top].val;
        else {
        /*</fb>*/
          attach_zval(json, json->stack[json->top].key,
            assoc, container_type);
        /*<fb>*/
        }
        /*</fb>*/
        if (!pop(json, Mode::OBJECT)) {
          s_json_parser->error_code = JSON_ERROR_STATE_MISMATCH;
          return false;
        }
        state = 9;
        break;
        /*
          [
        */
      case -6:
        if (!push(json, Mode::ARRAY)) {
          s_json_parser->error_code = JSON_ERROR_DEPTH;
          return false;
        }
        state = 2;

        if (json->top > 0) {
          Variant &top = json->stack[json->top].val;
          /*<fb>*/
          if (container_type == JSONContainerType::COLLECTIONS) {
            top = req::make<c_Vector>();
          } else if (container_type == JSONContainerType::HACK_ARRAYS) {
            top = Array::CreateVec();
          } else if (container_type == JSONContainerType::DARRAYS_AND_VARRAYS) {
            top = Array::CreateVArray();
          } else if (container_type == JSONContainerType::DARRAYS) {
            top = Array::CreateDArray();
          } else if (container_type == JSONContainerType::LEGACY_HACK_ARRAYS) {
            auto arr = staticEmptyVecArray()->copy();
            arr->setLegacyArray(true);
            top = arr;
          } else {
            top = Array::CreateDArray();
          }
          /*</fb>*/
          json->stack[json->top].key = copy_and_clear(*key);
          reset_type();
        }
        break;
        /*
          ]
        */
      case -5:
        {
          if (type != kInvalidDataType &&
               json->stack[json->top].mode == Mode::ARRAY) {
            Variant mval;
            json_create_zval(mval, *buf, type, options);
            auto& top = json->stack[json->top].val;
            if (container_type == JSONContainerType::COLLECTIONS) {
              collections::append(top.getObjectData(), mval.asTypedValue());
            } else {
              top.asArrRef().append(mval);
            }
            buf->clear();
            reset_type();
          }

          /*<fb>*/
          if (json->top == 1) z = json->stack[json->top].val;
          else {
          /*</fb>*/
            attach_zval(json, json->stack[json->top].key, assoc,
              container_type);
          /*<fb>*/
          }
          /*</fb>*/
          if (!pop(json, Mode::ARRAY)) {
            s_json_parser->error_code = JSON_ERROR_STATE_MISMATCH;
            return false;
          }
          state = 9;
        }
        break;
        /*
          "
        */
      case -4:
        switch (json->stack[json->top].mode) {
        case Mode::KEY:
          state = 27;
          std::swap(buf, key);
          reset_type();
          break;
        case Mode::ARRAY:
        case Mode::OBJECT:
          state = 9;
          break;
        case Mode::DONE:
          if (type == KindOfString) {
            z = copy_and_clear(*buf);
            state = 9;
            break;
          }
          /* fall through if not KindOfString */
        default:
          s_json_parser->error_code = JSON_ERROR_SYNTAX;
          return false;
        }
        break;
        /*
          ,
        */
      case -3:
        {
          Variant mval;
          if (type != kInvalidDataType &&
              (json->stack[json->top].mode == Mode::OBJECT ||
               json->stack[json->top].mode == Mode::ARRAY)) {
            json_create_zval(mval, *buf, type, options);
          }

          switch (json->stack[json->top].mode) {
          case Mode::OBJECT:
            if (pop(json, Mode::OBJECT) &&
                push(json, Mode::KEY)) {
              if (type != kInvalidDataType) {
                Variant &top = json->stack[json->top].val;
                object_set(
                  json,
                  top,
                  copy_and_clear(*key),
                  mval,
                  assoc,
                  container_type
                );
              }
              state = 29;
            }
            break;
          case Mode::ARRAY:
            if (type != kInvalidDataType) {
              auto& top = json->stack[json->top].val;
              if (container_type == JSONContainerType::COLLECTIONS) {
                collections::append(top.getObjectData(), mval.asTypedValue());
              } else {
                top.asArrRef().append(mval);
              }
            }
            state = 28;
            break;
          default:
            s_json_parser->error_code = JSON_ERROR_SYNTAX;
            return false;
          }
          buf->clear();
          reset_type();
          check_non_safepoint_surprise();
        }
        break;

        /*<fb>*/
        /*
          : (after unquoted string)
        */
      case -10:
        if (json->stack[json->top].mode == Mode::KEY) {
          state = 27;
          std::swap(buf, key);
          reset_type();
          s = -2;
        } else {
          s = 3;
          break;
        }
        /*</fb>*/

        /*
          :
        */
      case -2:
        if (pop(json, Mode::KEY) && push(json, Mode::OBJECT)) {
          state = 28;
          break;
        }
        /*
          syntax error
        */
      case -1:
        s_json_parser->error_code = JSON_ERROR_SYNTAX;
        return false;
      }
    } else {
      /*
        Change the state and iterate.
      */
      bool is_tsimplejson = options & k_JSON_FB_THRIFT_SIMPLE_JSON;
      if (type == KindOfString) {
        if (/*<fb>*/(/*</fb>*/s == 3/*<fb>*/ || s == 30)/*</fb>*/ &&
            state != 8) {
          if (state != 4) {
            utf16_to_utf8(*buf, b);
          } else {
            switch (b) {
            case 'b': buf->append('\b'); break;
            case 't': buf->append('\t'); break;
            case 'n': buf->append('\n'); break;
            case 'f': buf->append('\f'); break;
            case 'r': buf->append('\r'); break;
            default:
              utf16_to_utf8(*buf, b);
              break;
            }
          }
        } else if (s == 6) {
          if (UNLIKELY(is_tsimplejson)) {
            if (UNLIKELY(b != '0'))  {
              s_json_parser->error_code = JSON_ERROR_SYNTAX;
              return false;
            }
            escaped_bytes = 0;
          } else {
            escaped_bytes = dehexchar(b) << 12;
          }
        } else if (s == 7) {
          if (UNLIKELY(is_tsimplejson)) {
            if (UNLIKELY(b != '0'))  {
              s_json_parser->error_code = JSON_ERROR_SYNTAX;
              return false;
            }
          } else {
            escaped_bytes += dehexchar(b) << 8;
          }
        } else if (s == 8) {
          escaped_bytes += dehexchar(b) << 4;
        } else if (s == 3 && state == 8) {
          escaped_bytes += dehexchar(b);
          if (UNLIKELY(is_tsimplejson)) {
            buf->append((char)escaped_bytes);
          } else {
            utf16_to_utf8(*buf, escaped_bytes);
          }
        }
      } else if ((type == kInvalidDataType || type == KindOfNull) &&
                 (c == S_DIG || c == S_ZER)) {
        type = KindOfInt64;
        buf->append((char)b);
      } else if (type == KindOfInt64 && s == 24) {
        type = KindOfDouble;
        buf->append((char)b);
      } else if ((type == kInvalidDataType || type == KindOfNull ||
                  type == KindOfInt64) &&
                 c == S_DOT) {
        type = KindOfDouble;
        buf->append((char)b);
      } else if (type != KindOfString && c == S_QUO) {
        type = KindOfString;
        /*<fb>*/qchr = b;/*</fb>*/
      } else if ((type == kInvalidDataType || type == KindOfNull ||
                  type == KindOfInt64 || type == KindOfDouble) &&
                 ((state == 12 && s == 9) ||
                  (state == 16 && s == 9))) {
        type = KindOfBoolean;
      } else if (type == kInvalidDataType && state == 19 && s == 9) {
        type = KindOfNull;
      } else if (type != KindOfString && c > S_WSP) {
        utf16_to_utf8(*buf, b);
      }

      state = s;
    }
  }

  if (state == 9 && pop(json, Mode::DONE)) {
    s_json_parser->error_code = JSON_ERROR_NONE;
    return true;
  }

  s_json_parser->error_code = JSON_ERROR_SYNTAX;
  return false;
}